"Hydrogen line" . "spin-flip transition of a hydrogen atom's electron" . "21cm wave transition in atomic hydrogen" . "wavelength of 21 cm" . . "21 centimeter" . "21 cm hyperfine transition in neutral hydrogen" . "21-cm (1,420 MHz) region" . "21cm emission" . . "21-cm line emission" . "21 cm line" . "neutral atomic hydrogen (H I)" . "Hydrogen line" . . "21 cm spectral line" . "21 centimeters wavelength" . "HI 21-cm spectral line of neutral hydrogen" . "naturally emitted by hydrogen" . . "redshifted 21 cm radiation" . "21 centimeters" . "21 cm hydrogen" . "21-cm hydrogen line" . "hydrogen line" . "H I 21 cm line" . "21cm line" . "21 cm" . "neutral hydrogen line" . "21 cm radio emissions" . "atomic hydrogen emission" . . "21-centimeter H I line" . "21cm" . "1.4 GHz" . "21 cm Hydrogen line" . "21 cm observations" . "Hydrogen_line" . . . . "21cm emission line" . "collision with spin flip of an electron against a hydrogen atom" . "21-cm spectral line" . "21 cm line of atomic hydrogen" . "1420.40575177 MHz" . "21 cm emission" . "cosmos" . "21 cm spin line of neutral hydrogen" . "21-cm HI line" . "21 cm (1420 MHz)" . "Neutral Hydrogen (HI)" . "neutral atomic hydrogen (HI)" . "radio spectrum of hydrogen" . "21 cm line" . "HI emission" . "21-cm line" . "hydrogen line (21-centimeter line or H I line)" . "21cm line radiation" . "neutral hydrogen radio emission" . "21 cm radio waves" . "twenty-one centimeters" . "21-cm" . "HI" . "21cm Hydrogen line" . "H I" . . "neutral atomic Hydrogen emission" . "21 cm hydrogen line" . "cosmic 21-cm fluctuations" . "21 cm radiation" . "21 cm" . "HI 21 cm line" . "hydrogen gas" . "HI imaging" . "21 cm emissions" . "21-cm emission" . "21 cm H-I emissions" . "21cm hydrogen line" . "21 cm line of neutral hydrogen" . "neutral hydrogen" . "fundamental transition of the hydrogen atom" . "21-centimeter" . "emission frequency of neutral hydrogen" . "molecular hydrogen emission" . "spin-flip" . "HI sources" .